This invention relates to a drive circuit, and more particularly to a drive circuit comprised of insulated gate field effect transistors.
Insulated gate field effect transistors (hereinafter referred to as the "IGFETs") have been widely employed as switching means because their conduction can be easily controlled by a voltage applied to their gates. When a power source voltage (V.sub.DD) is operatively gated through the IGFET, it has been general that the IGFET is made conductive by a control signal having a level substantially equal to the power source voltage (V.sub.DD). Consequently, the IGFET operates in the saturated region and the output level (V.sub.out) outputted through the IGFET becomes a voltage lower than the power source voltage (V.sub.DD) by the threshold voltage (V.sub.th) of the IGFET. This relation is represented by the equation V.sub.out =V.sub.DD -V.sub.th. If this output level (V.sub.out) is used in order to drive another IGFET or other IGFETs of a subsequent stage circuit, the IGFET or IGFETs of the subsequent stage circuit receive a gate voltage of V.sub.out which is lower than the power source voltage (V.sub.DD) to operate in the saturated region. As a result, the IGFETs of the subsequent stage circuit can not assume a sufficient conductive state. Therefore, the output currents gated through the IGFETs of the subsequent stage are limited to a relatively small value. In order to obtain a desired output current, therefore, physical dimensions of the IGFETs of the subsequent stage circuit must be made larger.
For avoiding this disadvantage, the so-called "bootstrap circuit" has been employed to drive the IGFET in the non-saturated region for drawing out sufficient output voltage or current. In this bootstrap circuit, a potential rising in response to an input signal is added to the electric charge stored in a capacitor to obtain a potential higher than the power source voltage (V.sub.DD). Thus, the IGFET is driven in the non-saturated region by the higher potential. However, the potential stored across the capacitor decreases with time elapse due to leakage. It has therefore been difficult to maintain the higher potential throughout a relatively long time period. Furthermore, in the bootstrap circuit, the potential rising in response to the input signal is delayed with respect to the input signal at least by the period in which the capacitor is charged, and hence it has been difficult to attain high speed response.
In the field of digital audio which obtains an audio output signal on the basis of digital signal processing, the above-mentioned problem of the bootstrap circuit has become a serious problem. Namely, since audio signal contains a low frequency signal having a long cycle period, it is necessary to maintain the higher potential over the long cycle period. Accordingly, a reduction in level of the higher potential of the bootstrap circuit inevitably causes a reduction in level of the audio output signal. Furthermore, because the audio signal contains a variety of frequency components, response delay markedly deteriorates the reproduction of the signal waveform.